PAGING TIIDO: X68000 audio issue

Recently I purchased an X68000 Compact that has an audio issue where the PCM samples are clear (processed by the OKIM6295), but all YM2151 FM sounds are distorted and quiet. As most would expect, the capacitors needed to be replaced.

I replaced all capacitors with Nichicon tantalum capacitors, and the problem still exists.

I then replaced both of the MC3043 OP-AMPs, still not fixed.

I replaced the YM3012 DAC, still not fixed.

I replaced the YM2151, ditto.

Here is what the system sounds like. Note that all PCM sound effects are normal.


Any ideas? :?

I would check the power supply of the YM3012 (pin 1) and the opamp(s), if any is lower than 5V you'll be having such kind of problems. There's also Vref supply for opamps, which has to be half the power supply level, unless there's dual polarity supply present.
I am not familiar with X68000 much, I do know it is a complete beauty inside though ^^

OK, so this could be a power supply issue?

I have not recapped the PSU yet, but the motherboard has been completely overhauled.

The MC3403s are notorious for failing in these compacts, so I honestly thought that would have fixed it.

Power supply can cause such problems, but it is rather something else, but still power related. I would be checking all the voltage levels to see if anything is significantly off scale

Quote:

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Originally Posted by Robivy64

I purchased an X68000 Compact

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Can this run all X68000 games? I have never heard of the Compact.

The compact requires an external 5 1/4 drive to run retail games. The compact only has 3 1/2 floppy drives built in.

However, I have written a small batch file that formats 3 1/2 1.44 MB floppy drives as 1.20 MB for use in the X68000 compacts.

I don't know much about the X68000 but I have heard that the power supply is usually the first thing to go in those systems. Maybe you could try recapping the PSU or replacing it?

Don't copy that floppy! That system got a LOT of good games, it's too bad it wasn't a high-end game system that used carts.

Quote:

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Originally Posted by TmEE

I would check the power supply of the YM3012 (pin 1) and the opamp(s), if any is lower than 5V you'll be having such kind of problems. There's also Vref supply for opamps, which has to be half the power supply level, unless there's dual polarity supply present.
I am not familiar with X68000 much, I do know it is a complete beauty inside though ^^

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Tiido,

Input voltage levels are 5.06V

Here are the voltage readings i'm getting on the MC3403 OP-AMPs (at idle).

Pinout
http://circuits.datasheetdir.com/37/MC3403-pinout.jpg

MC3403 for sound effects (working)
Pin # Reading (V)
-----------------------
1 ..........3.78
2 ..........2.28
3 ..........2.54
4 ..........5.06
5 ..........2.55
6 ..........2.55
7 ..........2.65
8 ..........2.42
9 ..........2.42
10 ........2.03
11 ........0.00 (GND)
12 ........2.54
13 ........2.55
14 ........2.64

MC3403 for music (YM2151/YM3012, NOT working)
Pin # Reading (V)
-----------------------
1 ..........0.08
2 ..........0.08
3 ..........0.00
4 ..........5.06
5 ..........0.06
6 ..........0.56
7 ..........0.56
8 ..........0.14
9 ..........0.14
10 ........0.10
11 ........0.00 (GND)
12 ........0.03
13 ........0.55
14 ........0.55

As you can see, we have a huge difference in voltage levels here. Seeing as how my input voltage is 5.06, i'm thinking this op-amp might be bad. However, it is brand new. Any ideas? :?

You can replace the opamp with anything else though but I doubt its the opamp. Right now I am suspecting the Vref circuit that is supposed to generate 2.5V (or something close as you can see on SFX opamp) for virtual ground for opamps. Are there any X68000 schematics available ?

I can't find any schematics unfortunately :(

After poking around with a voltmeter, I discovered an SC815 transistor reading 0.00V on both the emitter and base, with 5.06V on the collector. The SC1815 transistors next to it are measuring 2.5-2.6V on the emitter.

Here is a photo of the audio system, with the transistor in suspect highlighted.
http://i32.photobucket.com/albums/d40/robivy64/rz3.jpg

OK, just replaced the transistor, same results.

You need to trace the circuit around the opamp with the faulty voltages. The VRef is probably done using 2 resistors from 5V to GND fed to voltage follower circuit (roughly)

:?: ...

Alright, I think I have sank enough time and money into this thing. I suspect that the board needs a couple new transistors (since there are several JP websites listing transistors as a common failure in these models). I think i'm going to cut my loss and sell this thing.

I just bought a refurbished X68000 Super on YJ, so i'll let someone else take a crack at this.